The present invention relates to a blood flow monitoring and control apparatus which is especially useful in connection with such treatments as blood dialysis, blood perfusion and blood diafiltration by utilizing only one connection point in the patient's blood circulatory system which is also referred to as the single needle technique.
The various methods for the detoxification of the blood require an external blood circulating system outside the human body. In other words, it is necessary to extract the blood with suitable means from the patient's inner blood circulatory system and to conduct the blood through the detoxification apparatus whereupon it must be returned back into the patient's internal blood circulatory system. Accordingly, normally two connecting points are necessary in the patient's blood circulatory system in order to maintain a continuous blood flow. Such connecting points are normally established by inserting a cannula through the skin and into a blood vessel of the patient.
The penetration of the skin by the cannula is painful to the patient and since the cannulas have a substantial diameter, damage to the skin at the point of penetration cannot be avoided. In many instances it has become a serious problem to find repeatedly new points for penetration by the cannula, especially where a patient must undergo repeated treatments at short intervals. These problems resulted in the development of the so called single needle technique for the performance of which only a single penetration cannula is required.
The single penetration point is sufficient because under the single needle technique blood is withdrawn from the inner circular system and returned into such inner system in an alternating manner whereby a phase of withdrawal is followed by a phase of return and so forth. This operation is discontinuous and, as a result, the efficiency of the treatment is somewhat reduced. However, such reduction in the treatment efficiency may easily be compensated by extending the duration of the treatment.
Conventional devices mostly control the alternating withdrawal and return of blood, either by predetermining the duration of the continuous change between withdrawal and return, or by making such continuous change dependent on the pressure conditions prevailing in the external blood circulatory system. The first mentioned method, which employs a predetermined timing, is technically very simple. However, it has the disadvantage that the pressure conditions in the external blood circulatory system are subject to undefined changes or variations.
The pure timing control does not constitute an optimal solution according to present day opinions of the experts, because the blood pressure is of substantial importance for the efficiency of a blood cleaning treatment, since the blood pressure in the blood cleaning apparatus influences the passage of the liquid through the dialyzer or through the filter membrane. Such a disadvantage is avoided in a system controlled in response to pressure provided that the control is arranged in such a manner that the pressure changes continuously back and forth between a fixed lower limit value and a fixed upper limit value, thereby defining a predetermined mean blood pressure value.
For several reasons it is possible in the pressure controlled system to establish merely a positive mean blood pressure value. Incidentally, this also applies to the time controlled system. This is so, because for safety purposes, the lower pressure limit may at best be a zero pressure value, because of the danger that contaminated liquid could possibly be sucked back into the internal circulatory system of the patient through a possibly defective dialyzer or filter membrane and due to the further danger that air might be sucked into the system due to leaks in the conduits of the external circulatory system. On the other hand, the upper pressure limit must be above the lower pressure limit by a certain minimal value in order to obtain a sufficient volume movement for each operational cycle. Thus, in such systems, only a positive mean blood pressure may be established. In certain instances, for example in the dialysis treatment of children, this requirement of the prior art systems may cause problems, because the liquid withdrawn from the circulatory system of a child must necessarily be very small.
Another danger occurs in both systems due to the continuous alternating loading of the dialysis or filter membrane, whereby the risk of a leak in the dialysis membrane is increased relative to a system in which the pressure load on the membrane is constant.
Yet a further disadvantage in the pressure controlled system alternating between blood withdrawal and blood return is seen in that the respective transported blood volume is not defined from the very start. Rather, the transported blood volume depends on the ability of the system to yield to the prevailing pressure. Such pressure yielding depends, for example, on the blood filling in the conventional air removal chamber. Depending on the manner of operating the system or on the quantity of air collected in such a chamber, the blood filling level in the air separation chamber may vary substantially. Furthermore, a volume shift or transport which is too small as well as a volume shift or transport which is too large in each cycle is also disadvantageous for medical reasons. In the first instance in which the volume shift is too small, the efficiency of the treatment is diminished. In the second instance in which the shifted volume of blood is too large, the volume variations in the internal blood circulatory system of the patient may also be too large.
It should also be mentioned here, that a break in the membrane or a larger leak in the conduit system acts as an infinite pressure yieldability. Stated differently, any switch-over is not accomplished and large quantities of blood may be lost through the leak unless additional safety measures are employed. However, such additional safety measures constitute a further increase of the apparatus expenses which in the pressure responsive control system is very large to begin with.
Due to the just mentioned disadvantages of prior art systems, the so called single needle technique has not been used on a large scale heretofore in spite of the advantages which this type of method does not provide in principle. Especially in the dialysis treatment in the patient's home, the single needle technique has hardly been used heretofore, mainly because the presently available systems are too uncertain in their function and also rather involved and hence it is difficult to operate these systems.